The most widely used rotation sensor units of this type include a housing in which the outer race of a rolling bearing is fitted, a shaft supported by the rolling bearing, and a magnetic encoder for detecting the rotation of the shaft. When using a magnetic encoder as a rotation sensor, if magnetic fluxes of an external magnetic field enter the magnetic sensor element of the magnetic encoder, its measurement accuracy decreases. Thus, the magnetic sensor element is typically mounted in a magnetic shield (JP Patent Publication 62-229026A).
For example, the rotation sensor unit disclosed in JP Patent Publication 62-229026A includes a shaft having its one end protruding from a housing, a duplex bearing supporting the shaft, an encoder mounted on the outer periphery of the shaft at its portion protruding from the housing, and a magnetic sensor element supported on the housing at its one end so as to radially face the encoder. The rotation sensor unit further includes a magnetic shield cover enclosing the housing, magnetic sensor element and encoder, and a protective cover enclosing the magnetic shield cover. By supporting the shaft with the duplex bearing, it is possible to prevent displacement of the magnetic sensor element and the encoder relative to each other. The protective cover is of a rigid structure to protect the magnetic sensor element, the circuit board, etc. from outside. By providing the protective cover and the magnetic shield cover separately from each other, the protective cover can be made of a lightweight non-magnetic material such as die-cast aluminum for high productivity.
If the magnetic shield cover is omitted, the protective cover can be made of a rigid ferromagnetic material, i.e. cast iron to use the protective cover as a magnetic shield too.
In the case of a rotation sensor unit having a protective cover like the one disclosed in JP Patent Publication 62-229026A, because the magnetic sensor element can be protected by the protective cover, it is sometimes required that not only the protective cover but also the housing be made of die-cast aluminum or resin. But if the housing is made of a non-magnetic material such as resin or aluminum, magnetic fluxes of an external magnetic field pass through the housing, thus decreasing measurement accuracy of the magnetic encoder under the influence of an external magnetic field.
Further, if the protective cover is omitted or if the protective cover and the housing are made of a non-magnetic material, it is necessary to provide a dedicated magnetic shield cover, which increases the number of component parts. Particularly if the protective cover is omitted, and the housing is made of a non-magnetic material, it is necessary to provide a large-sized magnetic shield cover that needs a large installation space, which in turn increases the size of the entire rotation sensor unit. Also, smaller magnetic encoders are used today and it is required to minimize displacement of the magnetic sensor unit and the encoder relative to each other.